http://www.radioshack.com/product/index.jsp?productId=2062251

get 24 aa batteries at harbor freight for 5 dollars usually. with cheaper ff the batteries last about 18 hours or so. but with a 570 di humminbird they dont last probably but 6 hours or so.

I did the comparison last month and decide to use rechargeable AAs for light weight and flexible use. I have a garmin 140. Based on my calculation, my 12 AA batteries (2000mAh each) can gave me a continuous operation of 3.5 hours (with the each battery discharged by 50%, we don't want to fully discharge them). Two thing motivated me to use 12 batteries. 1) my fish finder operates at 12-18V 2) the voltage of my battery drops gradually. I got an 8-pack and a 4-pack battery holder from radioshack for around $6 and connected them in serial. I would say the total cost of using batteries is not less than the lead-acid battery you showed above. AAs also last shorter. The main advantage is mainly light weight and flexible use. Hope this helps.

I use the small 8 pack AA Battery Pack with rechargeable batteries. Last around twelve hours.

I use this battery and can usually get 3 to 5 trips between charges. It is physically smaller but does the same job as my old deer feeder batteries. And it's a lot less expensive.

http://www.futurlec.com/Marine_Solar_Battery_Charger.shtml

Here's a 10A 12V marine solar charge controller, would be needed between your array and your battery should you actually have enough output to overcome the draw of the FF.

The electronics guru at work suggested it. He also pointed out that because kayaks move and you cant always have 100% sun that you should expect lower than average actual output from the solar cells.

He said that sadly Solyndra made a panel that was perfect for marine use because it was curved(convex) and would provide better coverage. And well, that's no more...

Side note, my Humminbird 570 draws a puny 200ma... I was shocked, I was sure it was going to be a solid 500ma+